Many advances in both science and technology have begun with the discovery of new materials with new or unusual properties, Warren et al., Materials Science and Engineering, 50:149-198, 1981. Following such discoveries, new applications are developed for the materials and the material properties are optimized for these applications.
Specific structures or structural modifications of particular compounds improve desirable physical properties of the compounds and/or test observed structure-property correlations for future optimization of the compounds' properties.
Artificially structured solid-state materials, i.e., materials where the structural and chemical characteristics are planned for and controlled, lead to many advances in technology. The synthesis of artificially structured, solid-state compounds has been dominated by extended high-temperature reaction of elements or binary compounds due to the slow diffusion rates in solids, DiSalvo, Science, 247:649-655, 1990. Such extended high-temperature reactions lead to the thermodynamically most stable product. During these reactions, many intermediate compounds form before the system achieves the final, thermodynamically stable configuration of products. Additionally, many desirable but thermodynamically metastable solid-state compounds cannot be synthesized by such conventional methods.
There is a demand for artificially structured compounds with optimized or optimizable thermoelectric properties for various specific thermoelectric devices. Materials used today in thermoelectric devices, Bi.sub.2 Te.sub.3 --Sb.sub.2 Te.sub.3, alloys for refrigeration and silicon, Si--Ge, alloys for power generation at elevated temperatures, were developed in the early 1960's, Sales et al., Science, 272:1325-1328, 1996. The main drawback of current thermoelectric materials used in devices is their efficiency. A material's efficiency depends on the material's properties, generally measured by the dimensionless parameter ZT, known as the thermoelectric Figure of Merit, where T is the temperature and ##EQU1## where S is the Seebeck coefficient (also known as the thermopower), .rho. is the electrical resistivity, and .kappa. is the thermal conductivity of the material.
Many materials have been studied with the hope of optimizing each material's thermopower (and hence, the ZT value), and consequently optimizing the efficiency of corresponding thermoelectric devices. Theory shows that there is no fundamental maximum to the thermoelectric Figure of Merit, Goldsmid, Electronic Refrigeration, Pion, London, 1986; Wood, Rep. Prog. Phys., 51: 459, 1988. A thermoelectric Figure of Merit of ZT=3 would make thermoelectric refrigerators competitive with traditional compressor-based refrigerators, Sales et al., Science, 272:1325-1328, 1996.
Recently, a class of compounds known as "filled skutterudites" have provided some promising thermoelectric materials. The filled skutterudite compounds satisfy the formula: EQU RM.sub.4 X.sub.12
where R is La, Ce, Pr, Nd, or Eu; M is Fe, Ru, or Os; and X is P, As, or Sb. The filled skutterudites are structurally related to the mineral skutterudite, CoAs.sub.3, but contain the cation La, Ce, Pr, Nd, or Eu in a large, interstitial site within the crystalline structure. Jeitschko et al., Acta Crystallographica, B33:3401-3406, 1977. Filled skutterudite compounds having Figures of Merit as high as ZT=0.9.+-.0.2 have been synthesized, Sales et al., Science, 272:1325-1328, 1996.
Filled skutterudites are promising thermoelectric materials because they have low thermal conductivity while still maintaining good electrical conductivity. The low thermal conductivity is thought to result from a large thermal vibration amplitude of the cation, which is equivalent to having a nearly soft phonon mode in the crystal lattice. The large vibration amplitude, which is a result of the interstitial site being too large for the cation, causes scattering of phonons and therefore a low phonon contribution to the thermal conductivity. Electrical conductivity, however, occurs primarily in the antimony framework, and is largely decoupled from the soft phonon mode.
Difficulties in synthesis have prevented production and measurement of the thermoelectric properties of most of the known filled skutterudite compounds, Sales et al., Science, 272:1325-1328, 1996. Filled skutterudites are made by conventional synthesis techniques resulting in, inter alia, mixed phase powders containing unwanted binary compounds and excess antimony, Braun et al., Journal of the Less-Common Metals, 72:147-156, 1980. Moreover, many desirable kinetically stable but thermodynamically unstable skutterudite crystalline-structured compounds cannot be synthesized by conventional, extended high-temperature methods. The thermodynamically metastable (unstable) skutterudite crystalline-structured compounds are desirable because of their thermoelectric properties.
Conventional synthesis of compounds having a skutterudite crystal structure have involved extended annealing of the components at high temperatures (i.e., 550.degree. C. and higher), yielding thermodynamically stable skutterudite compounds. That is, all previously synthesized or known skutterudite compounds have been thermodynamically stable compounds that have the largest free-energy gain upon crystallization relative to other possible compounds. Thermodynamically metastable compounds, on the other hand, have a relatively higher free-energy configuration at ambient temperature (i.e., the compound is not in the configuration with the lowest possible free energy), but do not have sufficient thermal energy to transform into the equilibrium configuration of a thermodynamically stable compound. Thermodynamically metastable compounds cannot be formed at temperatures greater than about 400.degree. C.-500.degree. C. Above about 400.degree. C.-500.degree. C. any formed thermodynamically metastable skutterudite crystalline-structured compounds decompose exothermically, forming the thermodynamically stable mixture of binary compounds and elemental components.